1. Field of the Invention
Embodiments of the invention relate to coating systems, particularly for threaded connections, and, in one embodiment, to nanocomposite coatings for corrosion resistance.
2. Description of the Related Art
In oilfields, it is common practice to use metallic pipes to extract oil or gas from underground reservoirs or pools. This extraction technique generally requires the boring of a well in the earth and internal casement of the well with relatively large diameter metallic tubes, referred to as the casing, to provide structural stability to the well and prevent it from collapsing. Subsequently, once the necessary depth of the well is achieved, a string of metallic pipes of smaller diameter, referred to as the tubing, are placed in the well in order to pump the gaseous or liquid hydrocarbons to the surface. The pipes which form the tubing are joined in a string of defined length which is sufficient to reach the depth necessary for extraction. Both the casing and tubing strings are formed of pipe segments, which are joined together using threaded connectors.
During assembly of either of the casing or tubing pipes, galling in the threads and other surfaces of the pipes which are in sliding contact is a significant problem which may arise. It is common practice to use dope or grease on the surface of the threads of male and female elements of the threaded connection during assembly of the pipes in order to avoid galling. The dope typically employs small particles of heavy metals such as Pb or Cu.
The use of dope presents significant drawbacks, however. Notably, excessive concentrations of heavy metals can result in their accumulation in the bodies of humans and animals, causing serious illness. Furthermore, the heavy metals present in dope may possibly contaminate soil, groundwater, and ocean water, presenting an environmental hazard. Furthermore, in light of these hazards, strict regulations prohibiting discharge of heavy metals requires precautions when utilizing dope that increases the cost of its use.
In addition to the aforementioned issues, the use of dope necessitates other operations, such as cleaning and doping of the threaded connections in the field during use. These operations are expensive and time consuming, however, as they are labor intensive. Additionally, these operations expose personnel to safety risks, as they require exposure to moving pipes and often take place under adverse conditions.
Another potential risk in the use of dope is the danger of either “underdoping” or “overdoping.” In underdoping, insufficient dope is provided and/or the dope fails to distribute over the entire surface of the threaded connection, resulting in a failure to adequately inhibit galling. While application of excess dope can address underdoping, this approach risks overdoping, the condition of laying too much dope on the threads of the connectors. Overdoping has the potential consequence that, during assembly of the connector, the excess of dope cannot be evacuated through the end of the threaded portions of the pipe segments. The trapped dope can thus develop high pressure within the connector that results in plastic deformation of the pipe segments in the threaded portion. In extreme cases, such plastic deformation may cause the collapse of the male member of the joint, rendering the connector ineffective, and requiring replacement of the pipe segments and/or the sleeve.
Anti-galling solutions which do not employ dope have been examined to address these problems. In one aspect, international patent application WO2006/075774 to Imai, et al. discloses a coating composition containing a lower layer, comprising a solid lubricating powder and a binder, and an upper coating layer of a solid corrosion protective coating without solid particles. At the time of fastening the joint, the corrosion protective coating gradually wears in the contact portions due to the friction occurring during fastening, exposing the underlying solid lubricating coating which provides its lubricating action.
In another aspect, U.S. Pat. No. 5,253,902 to Petelot, et al. describes an anti-seizing coating for protection of metallic threaded tubes. The coating comprises a thin lead oxide layer, produced by oxidation of a lead layer, which is formed on an underlying layer of copper, nickel, or chromium, which is in turn formed upon the substrate.
In a further aspect, International Patent Application WO/2006/104251 to Goto, et al. discloses a threaded joint having improved galling and corrosion resistance having a lower layer of a viscous liquid or semisolid lubricating coating comprising at least wax and a fatty acid alkaline earth metal salt without heavy metals. The coating also comprises an upper layer of a dry solid coating formed from an aqueous resin coating composition.
Each of these approaches has drawbacks, however. The approach of Imai relies upon a relatively soft corrosion protective coating which is worn through to expose the lubricating layer. As a result, the mechanical durability of the corrosion protective layer is low and this layer may wear away over time in undesired areas, significantly reducing the corrosion protection ability of the coating. The approach proposed by Petelot employs heavy metals, such as lead and copper, resulting in the attendant environmental and toxic hazards discussed above. The approach proposed by Goto is also problematic in that the lower layer of a liquid or semisolid lubricating coating will be subject to the problems of overdoping or underdoping when applying the lower layer composition.
For at least the foregoing reasons, there is a need for protection systems which provide improved corrosion resistance, especially in the severe environments faced within the oil and gas production industries.